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I thought that the structure will be aromatic and stabilised because one of the lone pairs being delocalised with the help of conjugated double bond system but still only 3 π-bonds will be present at a time and 2 lone pairs, so the answer should be 8.

But the book says the answer is 10. Please explain.

2-methylene-2H-pyran

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    $\begingroup$ Get a better book. Why? It should be 8, as only one pair can be delocalised. $\endgroup$ – Mithoron Jan 27 at 16:26
  • $\begingroup$ Please quote and cite the book you have used. This is a typical case where the erratum should be applied. $\endgroup$ – Martin - マーチン Jan 29 at 13:58
  • $\begingroup$ My rule for a conjugated ring: when an atom in the ring has lone pairs but no ligand, a lone pair must replace the ligand. After that all remaining lone pairs may be counted in the conjugated system. So here you can count only one of the two lone pairs on oxygen as pi electrons. Note that because you have pi bonding outside the ring there is not a 4n+2 rule for aromaticity. $\endgroup$ – Oscar Lanzi Apr 20 at 21:21

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